The most recurrent familial cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) is the presence of an abnormal number of intronic GGGGCC (G4C2) repetitions in the C9orf72 gene, which has been proposed to drive ALS/FTD pathogenesis. This evidence concerns the gene C9orf72 and amyotrophic lateral sclerosis.